My company buys these markers for use on PVC and metal well casings. I've returned to sites over a year later to find my marks intact even when petroleum products have been smeared all over the marks. Typical "permanent markers" cannot withstand this type of abuse. Also, these marks have been made on PVC in vaults that have freezing water in them fo 4 months out of the year and again... I find them intact. Superb Product! I would recommend these for Surveyors, Engineers, Welders, Pipe Fitters, etc..